In vehicles having internal combustion engines, it can be beneficial to discontinue fuel injection to all or some of the engine cylinders during certain operating conditions, such as during vehicle deceleration or braking. For example, one or more cylinder fuel injectors may be selectively deactivated while the engine is rotating. Such an operation is also known as a deceleration fuel shut-off (DFSO) event. The larger the number of cylinders that are deactivated, and/or the longer the cylinders are deactivated, the greater the fuel economy improvement that can be achieved.
However, there may be poor drivability issues during an engine reactivation following the DFSO. As such, following fuel deactivation, the engine may start spinning towards rest. In addition, where the engine is turbocharged, the turbine may also start spinning towards rest. If a vehicle operator tips-in during the engine spin-down, the time taken to spool up the turbine may be substantial, and the operator torque demand may not be timely met. As such, this may degrade engine performance and vehicle driveability. In addition, while the engine spins down, air may continue to flow through the cylinders towards an exhaust catalyst causing catalyst cooling as well as oxidizing catalytic sites. When combustion is re-initiated, additional fuel may be required to reduce and reactivate the exhaust catalyst. During some conditions, this fuel penalty may even outweigh the fuel economy benefits of the DFSO.
The inventors herein have recognized that such drivability issues may be overcome in a boosted engine system that uses a binary flow turbine. In one example, engine performance may be improved by a method comprising adjusting a scroll valve coupled to an inlet of one scroll of a multi-scroll exhaust turbine responsive to an indication of engine deactivation. By closing the scroll valve during a DFSO, turbine speed can be maintained elevated for a duration, enabling faster turbine spool-up upon subsequent engine reactivation, and reducing catalyst cooling.
In one example, in response to a DFSO event, a scroll valve coupled to only an outer scroll of a multi-scroll exhaust turbine may be moved to a more closed position. For example, the scroll valve may be moved, gradually or immediately, to a fully closed position. The moving of the scroll valve may be based on the turbine speed so that the turbine speed follows a desired deceleration profile. The scroll valve may then be maintained at the closed (e.g., fully closed) position during a subsequent engine restart. As such, by closing the scroll valve, the exhaust manifold pressure can be increased, allowing the turbine to keep spinning at or above a threshold speed while the engine spins towards rest. By enabling the turbine speed to follow a desired speed profile, in response to a sudden request for torque, such as due to a tip-in during the engine spin-down, the turbine may be quickly spooled up and the boost levels can be elevated. This allows drivability issues during a DFSO to be reduced. In addition, by closing the scroll valve, the elevated exhaust manifold pressure may reduce fresh air flow through the engine, decreasing the extent of exhaust catalyst cooling and oxidation. As such, this also lowers the fuel penalty required for reducing the exhaust catalyst upon engine reactivation. Wastegate and EGR valve adjustments may be coordinated with the scroll valve adjustments to further improve turbine speed control.
In this way, scroll valve adjustments in a binary flow turbine can be advantageously used to improve turbine speed control during an engine deactivation as well as during the subsequent engine reactivation. By improving turbine speed control, boost performance of the engine during the reactivation is improved. In addition, losses in exhaust catalyst performance during the DFSO, due to catalyst cooling and oxidation, are lowered. Overall, engine performance and fuel economy is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.